Does the effectiveness of scar treatment vary from person to person?

This product is the scar treatment gel, Noscarna Gel.

When my husband had shingles recently, I applied this diligently, and the results were amazing—he didn't get a single scar. So I tried applying it to a scar on my thigh as well, but I was disappointed because it looked exactly the same even after a few days. I'm curious if it doesn't work on old scars, if it's specialized for acne, or if the effects vary from person to person!